Although thoracic aortic aneurysm and dissection (TAAD) are not as common as some forms of vascular diseases, e.g. coronary artery disease, it is a clinically insidious disease with high fatality. If untreated, rupture occurs in 70% of patients resulting in a 94% mortality rate, which makes it a high impact health problem at both population and individual levels. Appropriate surgical therapy dramatically improves long-term survival. However, when and how to treat these patients is still experience- rather than evidence-based clinical practice. TAAD will occur in an aorta if one or more of the following conditions are present: 1) original aortic structure is too weak;2) destructive forces are too powerful;3) repair mechanisms are not competent. While all three aspects are critical in the disease development, the central hypothesis to be tested in this project is that extracellular matrix (ECM) production and organization in aortic wall determine how aorta will respond to inadequate matrix protease activation, hence formation of aneurysm or dissection. We have developed four Specific Aims: (1) To investigate the role of ECM metabolism in patients with TAAD. We will examine qualitatively and quantitatively the collagen and elastin, enzymes involved in collagen and elastin synthesis and secretion, and degradation in aortic samples of TAAD patients and compare with normal aortas. (2) To examine the hypothesis that ECM production capacity by aortic smooth muscle cells (SMCs) may be compromised in TAAD patients. We will use in vitro model by culturing aortic SMCs from TAAD patients and comparing with SMCs of normal aortas. (3) To examine the role aortic adventitial fibroblasts in ECM production in aortas from normal control and TAAD patients. In addition to resident medial layer SMCs, adventitial fibroblasts may either migrate to medial layer or stay in adventitial layer and contribute to the overall ECM metabolism. We will follow the same experimental design as those in the Aim 2 and evaluate the potential differences in fibroblasts collected from normal and TAAD aortas. (4) To evaluate the involvement of arterial wall ECM production in aneurysm development in vivo. To provide evidence for the causal relationship between the ECM production and aneurysm-like changes in aortic wall, we will use single gene knockout or transgenic mouse models and measure the impact on quantitative changes in aortic wall. We will regenerate TAAD by local CaCl2 challenge, which activates matrix metalloproteinases (MMPs) and causes aortic aneurysms. Our current project will investigate this novel hypothesis that ECM production determines the susceptibility to TAAD development, in vivo (patient study and animal model) and in vitro (cell culture model). Our project will lead to the establishment of evidence-based guides for surgical and/or medical treatments of TAAD. With our unique access to a large number of TAAD patients and expertise in cell and animal models, our project is feasible.